Zinc-halide batteries were developed as devices for storing electrical energy. Traditional zinc-halide batteries (e.g., zinc-bromine batteries) employed bipolar electrodes disposed in a static, i.e., non-flowing, zinc-bromide aqueous solution. The process of charging and discharging electrical current in a zinc-halide battery is generally achieved through a reaction of redox couples like Zn2+/Zn(s) and X−/X2 in zinc halide electrolyte, where X is a halogen (e.g., Cl, Br, or I).
When the battery is charged with electrical current, the following chemical reactions occur:Zn2++2e−→Zn2X−→X2+2e−.Conversely, when the battery discharges electrical current, the following chemical reactions occur:Zn→Zn2++2e−X2+2e−→2X−.Additionally, in some batteries, polyhalide reactions may also occur. Some of these examples are described by the following:X3−+2e−→3X− orXn−+ne−→nX− for n≥3.
The polyhalide reactions pictured above can include reactions between like halogens, e.g. Br3, and reactions between non-like halogens, e.g., mixed halogens such as Br2Cl.
These zinc-halide storage batteries were typically configured in a bipolar electrochemical cell stack, wherein each electrode is disposed in an aqueous zinc salt electrolyte. However, the performance of these storage batteries was highly inefficient due to secondary reactions of the dissolved species in the aqueous electrolyte. For example, in solution, elemental bromine exists in equilibrium with bromide ions to form polybromide ions, Brm−, where m=3, 5, or 7. Secondary hydrolysis reactions are also problematic for these types of storage batteries when the electrolytes are formulated with excess free water, because bromate solids form, which in turn reduces the amount of available bromide/bromine that can under reduction or oxidation in the electrochemical cell.

Elemental bromine also possesses an increased vapor pressure that promotes hazardous pressure in the batteries. Furthermore, when aqueous zinc halide salts are ionized, zinc ions can exist as various complex ions and ion pairs, which promotes zinc dendrite formation and increased incidence of self-discharge in the batteries. To improve electrolyte durability in the batteries, halogen sequestration agents were added (e.g., quaternary ammonium salts or heteroaryl salts (e.g., pyridinium)); however, these sequestration agents typically possessed reduced solubility and reduced the stability of the electrolyte over numerous charge cycles.